This will allow embedders to actually instantiate the {DefaultJobHandle}
as suggested in the comment. Node currently implements {PostJob()} by
just returning a nullptr. After this change, it can use the new
{NewDefaultJobHandle} function and we can actually start using this API
in v8.

R=adamk@chromium.org

Bug: v8:10723
Change-Id: I4b31a640d0edc7e7207d1df95e683465dfaaaeff
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2304812Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68934}